According to Johan Mårtensson from Lund University, if you are learning new language quickly, it helps your brain to become bigger and increase its activity:
This finding came from scientists at Lund University, after examining young recruits with a talent for acquiring languages who were able to speak in Arabic, Russian, or Dari fluently after just 13 months of learning, before which they had no knowledge of the languages.
After analyzing the results, the scientists saw no difference in the brain structure of the control group. However, in the language group, certain parts of the brain had grown, including the hippocampus, responsible for learning new information, and three areas in the cerebral cortex.
And there is more:
One particular study from 2011 provided evidence that Alzheimer's was delayed 5 years for bilingual patients, compared to monolingual patients.